Laser printers and Xerographic printers utilize charged toner to render an image on paper. Typically, the toner is charged and deposited on a charged drum in an image pattern. Toner that is to form part of the image is transferred to a charged sheet of paper or other material to be imaged. Toner that does not form part of the image is captured to be reused.
As toner is repeatedly charged, exposed to heat, and reused, the toner uniformity declines. In particular, the charge carrying capacity of the toner particles begins to change such that different toner particles have different physical characteristics resulting in different charge carrying capacities. Toner non-uniformity results in lower quality printer output.
Unfortunately, determining the optimum time to replace the toner is difficult. Counting the number of pages output is an inaccurate measure, because toner intensive printed pages will result in a quick consumption of toner and prevent substantial amounts of reused toner from being generated. Replacing the toner too early is wasteful and also increases printer operating costs. However, waiting too long to replace the toner can result in printer output degradation. Printing unusable output also results in waste.
Thus an improved method of measuring the decline in toner uniformity, or in general, determining when toner should be replaced, is needed.